From help-octave-request at bevo dot che dot wisc dot edu  Tue Jan 19 01:37:46 1999
Subject: RE: diff(x) question
From: Van den Eynde Gert <gvdeynde at SCKCEN dot BE>
To: help-octave at bevo dot che dot wisc dot edu
Date: Tue, 19 Jan 1999 08:37:32 +0100


> 
> t=[1:.1:10]
> x=sin(t)
> and I want to plot the derivative of sin(t) (but I want to compute it)
> 
> I tried
> y=diff(x)
> 
> and then plot(t,x,t,y) yields me an error message that states the
> the vector sizes must be the same.. so I did
> plot(t,x,[1.1:.1:10],y)
> 
> and it worked, but the y is really small and never goes beyond .1 in
> value.  Do you think you could point me in the right direction?
> I guess I'm differentiating wrong, how should this really be done?
> 

Diff computes forward differences. Not the derivative. If you want to 
approximate the derivative by using forward differences you have to 
divide by the step h with which you make your function discrete.
In this case h = .1


df          f(t+h) - f(t)
-- (t) =~  --------------- + O(h)
dt                h



Sincere greetings,
Gert Van den Eynde

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